Abstract: The hybrid simulation system (HSS) at the University of California, Berkeley (UCB), namely nees@berkeley, developed and used in this study, is presented in details. The validation of this HSS is sought through testing steel cantilever columns with predictable structural response that is verifiable by purely numerical simulation of the experiment. Two procedures are developed and implemented in the HSS with the aim of enhancing the accuracy and reliability of the online, i.e. pseudo-dynamic, test results. The first procedure aims at correcting the experimental systematic error in executing the displacement command signal. The error is calculated as the difference between the command and the feedback signals and correlated with the actuator velocity using the least squares method. A feed-forward scheme for error compensation is devised leading to a more accurate execution of the test. The second procedure employs mixed variables with mode switching between displacement and force controls. The newly derived force control algorithm is evaluated using a parametric study to assess its stability and accuracy. The implementation of the mixed variables procedure is designed to adopt force control for high stiffness states of the structural response and displacement control otherwise, where the resolution of the involved instruments may favor this type of mixed variables control. The application of the two procedures is carried out successfully on the experimental structures described below and several implementation strategies are developed within the HSS. Two experimental test structures are considered in this study to demonstrate different aspects of the developed procedures in the HSS. These test structures are: (1) Reinforced concrete (RC) frames with and without unreinforced masonry infill wall as substructures of a five-story infilled RC building; and (2) Wood shear walls of the first story of a two-story wood house-over-garage. The design, construction and instrumentation of the test structures and their experimental setup designs are discussed. Each test structure is idealized into a lumped mass system and the formulation of the governing equations of motion based on this idealization is discussed. The structural performance of the two test structures under seismic loading is evaluated using the developed HSS. Furthermore, the two test structures have the common feature of being large substructures of shaking table experiments conducted at UCB and accordingly, a comprehensive comparative study is conducted between the test results of the two testing methods, namely hybrid simulation and the shaking table. Several sources of differences and their contributions to the discrepancies between the results of the two testing methods are identified.